1. Field of the Invention
This invention relates to solid diffusion dopants for semiconductors and methods of making the same. More particularly, the invention pertains to solid diffusion sources for the phosphorus doping of semiconductors and methods of making the same.
2. Description of the Prior Art
At present, the so-called open tube method is mainly employed for the diffusion of phosphorus into silicon semiconductors and phosphorus containing substances such as red phosphorus, P.sub.2 O.sub.5, POCl.sub.3, PH.sub.3 etc. are used as phosphorus diffusion sources. Particularly, POCl.sub.3, and PH.sub.3 are employed in many cases.
In the diffusion employing such impurity sources, an accurate control of the pressure of vapor from the diffusion source, or the flow rate of a gas when it is used as the diffusion source, is required for controlling the concentration of phosphorus diffusing into the silicon. And this necessitates a precise control of the temperature of the diffusion source, the flow rate of the carrier gas, etc.
Conventional doping methods employing liquid diffusion sources are briefly, as follows. Only vapor emanating from the diffusion source, heated at a temperature, below 600.degree. C, or a mixed gas of the vapor and a very small amount of oxygen or a large amount of inert gas, is introduced into a doping chamber kept at a temperature ranging from 750.degree. to 1200.degree. C to effect therein the diffusion of phosphorus into silicon semiconductors.
In this case, many silicon slices or wafers are usually arranged in the doping chamber for the purpose of enhancement of the doping efficiency. In such a doping procedure, however, the concentration of phosphorus diffusing into the silicon wafers may greatly differ on the sides of gas inlet and outlet in some cases. This is caused by a non-uniform flow of gas in the doping chamber, so that a uniform flow of gas is necessary for uniform diffusion of phosphosphorus. However, it is very difficult to establish such a uniform flow of gas. Even under the same doping conditions, dispersion is noted in the phosphorus concentration among doping lots or silicon wafers of the same lot or in each silicon wafer.
Further, the gas diffusion sources such as PH.sub.3, PBr.sub.3, etc. have not only the same disadvantages as the liquid diffusion sources mentioned above but also a disadvantage of high toxicity. Moreover, solid red phosphorus has a defect of fluctuation in composition due to a thermal history of diffusion and P.sub.2 O.sub.5 has high hygroscopicity which is likely to carse a fluctuation in the vapor pressure. In any case, the phosphorus concentration in the silicon varies widely.
To overcome such shortcomings described above, wafer-shaped, solid dopants, which are equal in size to or a little larger than the silicon wafers, are used in the diffusion of boron in silicon semiconductor. With this method, the dopants and the silicon wafers are alternately disposed in parallel but closely spaced relationship to each other and then simultaneously placed in a doping chamber precisely controlled in temperature. In the doping chamber, a gas containing boron, generated from the dopants reacts with the silicon and the boron diffuses into the silicon wafers. With this method, all the silicon wafers make contact with the gas containing boron, generated from the dopants disposed adjacent thereto, and this eliminates the possibilities of dispersion in the diffused boron concentration resulting from the difference in position among the silicon wafers. Thus, uniform diffusion is easily achieved
Also in the case of phosphorus, if such a substance is available which remains solid at such a diffusion temperature as mentioned above and can be disposed adjacent to the silicon wafers and which generates a gas containing phosphorus at the diffusion temperature, phosphorus will also make useful diffusion dopants. Recently, inventions of solid diffusion dopants, made for the abovesaid purpose, have been patented.
One of such solid dopant sources is indicated in U.S. Pat. No. 3,849,344, in which the dopant contains a compound SiO.sub.2.P.sub.2 O.sub.5 or 2SiO.sub.2.p.sub. 2 O.sub.5 and the others are proposed in U.S. Pat. Nos. 3,841,927 and Published Application No. B 351,348, in which the dopants contain Al(PO.sub.3).sub.3. However, these solid dopants are all those which are suitable for the diffusion of the so-called high phosphorus concentration region that the surface phosphorus concentration in silicon wafers after predeposition is 10.sup.20 to 10.sup.21 atoms/cm.sup.3, and, at a temperature suitable for the diffusion of a region having a surface phosphorus concentration of less than about 10.sup.20 atoms/cm.sup.3, a sufficient amount of phosphorus-containing gas is not generated from the dopants and the surface phosphorus concentration in the silicon wafers greatly varies with the place.